# std::invoke, std::invoke_r

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< cpp‎ | utility‎ | functional

Defined in header `<functional>` |  |
template< class F, class... Args > std::invoke_result_t<F, Args...> invoke( F&& f, Args&&... args ) noexcept(/* see below */); | (1) | (since C++17) (constexpr since C++20)
template< class R, class F, class... Args > constexpr R invoke_r( F&& f, Args&&... args ) noexcept(/* see below */); | (2) | (since C++23)

1) Invoke the Callable object f with the parameters args as by `INVOKE`(std::forward<F>(f), std::forward<Args>(args)...). This overload participates in overload resolution only if std::is_invocable_v<F, Args...> is true.
2) Invoke the Callable object f with the parameters args as by `INVOKE<R>`(std::forward<F>(f), std::forward<Args>(args)...). This overload participates in overload resolution only if std::is_invocable_r_v<R, F, Args...> is true.

### Parameters

f | - | Callable object to be invoked
args | - | arguments to pass to f

### Return value

1) The value returned by f.
2) The value returned by f, implicitly converted to `R`, if `R` is not (possibly cv-qualified) void. None otherwise.

### Exceptions

1)
`noexcept` specification:
noexcept(std::is_nothrow_invocable_v<F, Args...>)
2)
`noexcept` specification:
noexcept(std::is_nothrow_invocable_r_v<R, F, Args...>)

### Possible implementation

invoke (1)
    namespace detail
    {
        template<class>
        constexpr bool is_reference_wrapper_v = false;
        template<class U>
        constexpr bool is_reference_wrapper_v<std::reference_wrapper<U>> = true;

        template<class T>
        using remove_cvref_t = std::remove_cv_t<std::remove_reference_t<T>>;

        template<class C, class Pointed, class Object, class... Args>
        constexpr decltype(auto) invoke_memptr(Pointed C::* member, Object&& object,
                                               Args&&... args)
        {
            using object_t = remove_cvref_t<Object>;
            constexpr bool is_member_function = std::is_function_v<Pointed>;
            constexpr bool is_wrapped = is_reference_wrapper_v<object_t>;
            constexpr bool is_derived_object = std::is_same_v<C, object_t>
                                            || std::is_base_of_v<C, object_t>;

            if constexpr (is_member_function)
            {
                if constexpr (is_derived_object)
                    return (std::forward<Object>(object) .* member)
                               (std::forward<Args>(args)...);
                else if constexpr (is_wrapped)
                    return (object.get() .* member)(std::forward<Args>(args)...);
                else
                    return ((*std::forward<Object>(object)) .* member)
                               (std::forward<Args>(args)...);
            }
            else
            {
                static_assert(std::is_object_v<Pointed> && sizeof...(args) == 0);
                if constexpr (is_derived_object)
                    return std::forward<Object>(object) .* member;
                else if constexpr (is_wrapped)
                    return object.get() .* member;
                else
                    return (*std::forward<Object>(object)) .* member;
            }
        }
    } // namespace detail

    template<class F, class... Args>
    constexpr std::invoke_result_t<F, Args...> invoke(F&& f, Args&&... args)
        noexcept(std::is_nothrow_invocable_v<F, Args...>)
    {
        if constexpr (std::is_member_pointer_v<detail::remove_cvref_t<F>>)
            return detail::invoke_memptr(f, std::forward<Args>(args)...);
        else
            return std::forward<F>(f)(std::forward<Args>(args)...);
    }
invoke_r (2)
    template<class R, class F, class... Args>
        requires std::is_invocable_r_v<R, F, Args...>
    constexpr R invoke_r(F&& f, Args&&... args)
        noexcept(std::is_nothrow_invocable_r_v<R, F, Args...>)
    {
        if constexpr (std::is_void_v<R>)
            std::invoke(std::forward<F>(f), std::forward<Args>(args)...);
        else
            return std::invoke(std::forward<F>(f), std::forward<Args>(args)...);
    }

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_invoke` | `201411L` | (C++17) | `std::invoke`, (1)
`__cpp_lib_invoke_r` | `202106L` | (C++23) | `std::invoke_r`, (2)

### Example

Run this code

    #include <functional>
    #include <iostream>
    #include <type_traits>

    struct Foo
    {
        Foo(int num) : num_(num) {}
        void print_add(int i) const { std::cout << num_ + i << '\n'; }
        int num_;
    };

    void print_num(int i)
    {
        std::cout << i << '\n';
    }

    struct PrintNum
    {
        void operator()(int i) const
        {
            std::cout << i << '\n';
        }
    };

    int main()
    {
        std::cout << "invoke a free function: ";
        std::invoke(print_num, -9);

        std::cout << "invoke a lambda: ";
        std::invoke([](){ print_num(42); });

        std::cout << "invoke a member function: ";
        const Foo foo(314159);
        std::invoke(&Foo::print_add, foo, 1);

        std::cout << "invoke (i.e., access) a data member num_: "
                  << std::invoke(&Foo::num_, foo) << '\n';

        std::cout << "invoke a function object: ";
        std::invoke(PrintNum(), 18);

    #if defined(__cpp_lib_invoke_r)
        auto add = [](int x, int y){ return x + y; };
        std::cout << "invoke a lambda converting result to float: ";
        auto ret = std::invoke_r<float>(add, 11, 22);
        static_assert(std::is_same<decltype(ret), float>());
        std::cout << std::fixed << ret << "\ninvoke print_num: ";
        std::invoke_r<void>(print_num, 44);
    #endif
    }

Possible output:

    invoke a free function: -9
    invoke a lambda: 42
    invoke a member function: 314160
    invoke (i.e., access) a data member num_: 314159
    invoke a function object: 18
    invoke a lambda converting result to float: 33.000000
    invoke print_num: 44

### See also

mem_fn (C++11) | creates a function object out of a pointer to a member (function template)
result_ofinvoke_result (C++11)(removed in C++20)(C++17) | deduces the result type of invoking a callable object with a set of arguments (class template)
is_invocableis_invocable_ris_nothrow_invocableis_nothrow_invocable_r (C++17) | checks if a type can be invoked (as if by std::invoke) with the given argument types (class template)
apply (C++17) | calls a function with a tuple of arguments (function template)

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